Characterization of a commercialized SERS-active substrate and its application to the identification of intact Bacillus endospores

Abstract

Surface-enhanced-Raman-spectroscopy (SERS) can be made an attractive approach for the identification of Raman-active compounds and biological materials (i.e., toxins, viruses, or intact bacterial cells or spores) through development of reproducible, spatially uniform SERS-active substrates. Recently, reproducible (from substrate to substrate), spatially homogeneous (over large areas) SERS-active substrates have been commercialized and are now available in the marketplace. Scanning electron microscopy and high-resolution, tapping-mode atomic force microscopy have been used to analyze these novel plasmonic surfaces for topographical consistency. Additionally, we have assessed, by wavelength-tunable microreflectance spectrometry, the spatial distribution of the localized surface plasmon resonance (LSPR) across a single substrate surface as well as the LSPR lambda(MAX) variance from substrate to substrate. These analyses reveal that these surfaces are topologically uniform with small LSPR variance from substrate to substrate. Further, we have utilized these patterned surfaces to acquire SERS spectral signatures of four intact, genetically distinct Bacillus spore species cultivated under identical growth conditions. Salient spectral signature features make it possible to discriminate among these genetically distinct spores. Additionally, partial least squares, a multivariate calibration method, has been used to develop personal-computer-borne algorithms useful for classification of unknown spore samples based solely on SERS spectral signatures. To our knowledge, this is the first report detailing application of these commercially available SERS-active substrates to identification of intact Bacillus spores.

Surface-Enhanced Raman Scattering (SERS) and Surface-Enhanced Resonance Raman Scattering (SERRS): A Review of Applications

“…There are a very wide range of possible SERS applications including its use to identify many analytes such as DNA bases, 47 explosives, 48 therapeutic agents, 49 drugs of abuse, 50 food additives, 51 and cells and spores. 52 As examples, three applications illustrating the potential of the technique are highlighted.…”

“…The data is usually analyzed by methods such as partial least squares (PLS) and principal component analysis (PCA) and can discriminate between genetically different species of intact bacillus spores. 52 As an illustration of the differences between species, Fig. 7 shows the spectra of three distinct cell types and their classification using a standard PCA method.…”

Detecting single Bacillus spores by surface enhanced Raman spectroscopy

“…These problems can be solved by using a much more sensitive vibrational spectroscopic method, surface enhanced Raman spectroscopy (SERS), which could reach the limit of detection (LOD) to a single spore or cell [9][10][11], molecules down to the parts per billion (ppb) level or possibly a single molecule [12]. Raman signals can be enhanced by more than a million times when the probed molecules are attached to metallic nanostructures (typically, Au, Ag, Cu, Pt or Pd), which allow the enhanced Raman scattering takes place in high local optical fields of these structures due to ''electromagnetic field enhancement'' and ''chemical enhancement'' of signals [12,13].…”

Characterization of Bacilli Spores by Surface-Enhanced Raman Spectroscopy, a Fast and Reliable Technique for Early Warning of Biological Threats

“…The sampling laser, emitting at 785 nm (linewidth < 0.3 nm), with 150 mW power is focused on the sample giving a laser spot of about 35-40 μm diameter. SERS sensors (Klarite®, Renishaw Diagnostics inc.) are composed of regular arrays of inverted pyramidal pits realized depositing a sputtered gold layer on a silicon substrate with an ordered nanostructure produced by electron beam lithography [4]. The regularity of the nanostructure guarantees a uniform enhancement of the weak Raman signal over the whole excited area.…”